The present invention relates to a packaging machine for continuously producing sealed packages of a pourable food product, and having a capacitive level sensor.
Many pourable food products, such as fruit juice, UHT milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik or Tetra Brik Aseptic (registered trademarks), which is formed by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene, and, in the case of aseptic packages for long-storage products, such as UHT milk, also comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package contacting the food product.
As is known, such packages are made on fully automatic packaging units, on which a continuous tube is formed from the packaging material supplied in strip form; the strip of packaging material is sterilized on the packaging unit itself, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form a tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed and cut at equally spaced cross sections to form pillow packs, which are then folded mechanically to form the finished, e.g. substantially parallelepiped-shaped, packages.
More specifically, the food product is fed from the usual storage tank into the tube of packaging material along a fill conduit extending inside the tube of packaging material and having a flow-regulating solenoid valve.
To ensure a substantially constant level of the food product inside the tube of packaging material during formation of the packages, known packaging machines are also normally provided with level-maintaining devices comprising a level sensor for determining the level of the food product inside the tube; and a control device for controlling the flow-regulating solenoid valve, and operating on the basis of the signal from the level sensor.
Numerous types of level sensors are known. Some feature a float housed inside the tube of packaging material, and the position of which is determined either by means of mechanical devices also housed inside the tube of packaging material, or by means of Hall-effect sensors located outside the tube of packaging material and which detect the presence of magnetic elements carried by the float.
Another type features a conducting rod partly immersed in the food product inside the tube of packaging material, and the exposed end of which is connected to an electric circuit located outside the tube, and to which the fill conduit is also connected. In this solution, the food product, being conductive, electrically connects the immersed portion of the rod and the fill conduit, which are thus connected in series within the electric circuit to which they are connected; and, since the actual resistance of the rod, and hence the values of electric quantities in the circuit, such as current flow, depend on the level of the food product inside the tube of packaging material, this is therefore determined on the basis of the values of said electric quantities.
Another type of level sensor is described, for example, in U.S. Pat. No. 4,675,660 filed by TETRA DEV-CO Consorzio di Studio e Ricerca Industriale, and operates on the principle of creating energy waves inside the fill conduit using a transducer housed inside the tube of packaging material and contacting the fill conduit. The energy waves are transmitted to the food product inside the tube of packaging material, and may therefore be detected and so converted as to indicate the level of the food product inside the tube of packaging material.
A common drawback of all the level sensors described above is the use of componentsxe2x80x94such as floats, mechanical devices, rods, transducersxe2x80x94housed inside the tube of packaging material, and which, being in contact with the food product for packaging; require regular thorough cleaning to ensure strictly hygienic packaging conditions.
Moreover, on account of the form and location of the components inside the tube of packaging material, the actual cleaning operation is often a long, painstaking job.
European Patent EP-B1-0681961, filed by the present Applicant, describes a level sensor designed to eliminate the aforementioned drawback typically associated with level sensors of the type described above.
The level sensor in question operates on the principle of determining the level of the food product inside the tube of packaging material using a temperature-detecting device located outside the tube of packaging material and comprising a number of temperature sensors located successively along the tube; and the level of the food product inside the tube of packaging material is determined on the basis of the relationship between the number of temperature sensors detecting a surface temperature of the tube affected by the food product, and the number of temperature sensors detecting a surface temperature of the tube not affected by the food product.
Featuring a large number of temperature sensors, however, the level sensor described in the above patent is fairly complex, both to produce and in terms of computation, by requiring more or less complex processing of the various temperature sensor signals.
Moreover, using the above level sensor, the flow-regulating solenoid valve is controlled, not in real time, but with a certain delay correlated to the thermal inertia of the packaging material of the tube. That is, since the intrinsic thermal inertia of the packaging material is other than zero, the effect of a variation in food product level on the temperature of the tube, as opposed to be being determined in real time by the temperature sensors, can only be determined some time after the instant in which it occurs, thus inevitably also affecting control of the flow-regulating solenoid valve and of the food product level.
It is an object of the present invention to provide a packaging machine featuring a level sensor located outside the tube of packaging material, and which is straightforward and cheap to produce, and provides for real-time detecting variations in food product level.
According to the present invention, there is provided a packaging machine for continuously producing sealed packages of a pourable food product from a tube of heat-seal sheet packaging material fed along a vertical path and filled continuously with said food product by means of a fill conduit extending inside said tube; said packaging machine comprising level-sensor means for detecting the level of said food product inside said tube; and being characterized in that said level-sensor means comprise capacitive level-sensor means located outside said tube.